It is very difficult to stably disperse carbon black in an aqueous medium at a high concentration since carbon has hydrophobicity and low wettability with water. This is because the surface of carbon black has a very small amount of hydrophilic functional groups (e.g., acidic hydroxyl groups such as a carboxyl group or a hydroxyl group) that exhibit high affinity to an aqueous medium (e.g., water molecules).
Attempts have long been made to improve the dispersibility of carbon black in water by oxidizing carbon black to form hydrophilic functional groups on the surface of the carbon black.
For example, Patent Document 1 (JP-A-48-018186)proposes a method that oxidizes carbon black using a hypohalite aqueous solution, and Patent Document 2 (JP-A-57-159856) proposes a method that oxidizes carbon black using low-temperature oxygen plasma. However, a carbon black aqueous dispersion that exhibits excellent blackness (print density) and dispersibility in water; and may suitably be used in an aqueous black pigment (colorant) ink such as an inkjet printer black ink, is still desired at present.
A method for producing an aqueous ink that attempts to improve dispersibility in water by utilizing carbon black that is subjected to a low degree of oxidation in combination with a coupling agent or a surfactant and the like, has been proposed (e.g., Patent Document 3 (JP-A-4-189877)). However, it is difficult to stably maintain the dispersibility for a long time due to oxidation or decomposition (i.e., a change in properties) of the surfactant and the like caused by a change in temperature or a change with the passage of time.
Carbon black may be surface-treated white improving dispersibility by finely grinding carbon black in water using glass beads, and oxidizing the carbon black using a hypohalite. According to this method, however, the grinding effect may be reduced due to the buoyancy acting on the glass beads in water, and it is difficult to uniformly form functional groups on the surface of the carbon black since an active site is formed to only a small extent.